Guckel et al. in U.S. Pat. No. 4,744,863 disclose the use of a sealed cavity semiconductor pressure transducer employing a flexible, polysilicon deformable diaphragm that is formed over a sacrificial oxide post on a semiconductor substrate. The sacrificial post includes a plurality of web-like appendages extending along the substrate beyond the periphery of the post. After the diaphragm material is conformally deposited upon the post, the periphery of the diaphragm material is etched away to reveal the sacrificial oxide. The sacrificial oxide is then etched away beneath the flexible diaphragm in order to define a diaphragm cavity. The holes formed in the perimeter of the diaphragm after the sacrificial oxide is etched away are then sealed in order to form the diaphragm cavity.
The preferred embodiment teaches the use of resistive and other similar passage sensors that are deposited on the flexible diaphragm. The resistance of these resistors will change in proportion to the deformation of the flexible diaphragm as the ambient pressure changes with respect to the pressure sealed within the diaphragm cavity. At column 6, in lines 47-63, mention is also made of using a structure for carrying two plates of a capacitive sensor such that the capacitance will change in response to the deflection of the diaphragm.
In contrast to the prior art, it is an object of the present invention to utilize a single crystal silicon diaphragm that is epitaxially formed on an SOI (silicon-on-insulator) substrate, where the thickness of the sacrificial oxide layer on the SOI wafer is generally equal to the thickness of the diaphragm cavity to be formed. The use of a highly controllable silicon dioxide coating on the SOI wafer yields a capacitor having a moderate operative capacitance and a minimum parasitic capacitance.
Furthermore, the use of a thin silicon film as the starting material for the construction of the diaphragm assures that the sensor will include all of the properties and advantages of a single crystal diaphragm. The mechanical properties of single crystal diaphragms are much more predictable than those of other silicon materials and structures, and therefore sensors having single crystal diaphragms can be fabricated with much tighter tolerances and performance specifications. And finally, the present fabrication process does not require the bonding of patterned structures for the diaphragm.
Further objects, features, and advantages of the invention will be apparent from references to the following detailed specification when taken in conjunction with the attached drawings.